Over the past several years, attention has been directed on a special carbon fiber called a “carbon nanofiber” as a reinforcing material, and methods for utilizing this fiber have been proposed. A carbon nanofiber is a sheet rolled into a cylindrical shape containing carbon atoms arranged in the form of a hexagonal mesh, and having a diameter of 1.0 nm (nanometers) to 150 nm. Carbon nanofibers are also called carbon nanotubes, and shall be generically referred to as carbon nanomaterials hereinafter. Carbon nanomaterials are from several to 100 μm in length.
Since carbon nanomaterials are very small, they tend to aggregate and do not readily mix uniformly with a matrix metal used as a base material.
Accordingly, techniques for mixing a ceramic material or a carbon nanomaterial with a low-melting metallic material that is in a semi-molten state and for creating a composite are known, as disclosed in JP 06-238422 A and JP 2004-136363 A.
According to the method for manufacturing a composite metal member disclosed in JP 06-238422 A, a metal material with which a reinforcing material has been admixed is heated and dissolved into a semi-molten state, and then stirred and mixed. The material is then further heated and melted into a liquid state, and thereafter stirred and mixed. The material is then returned to a semi-molten state, again stirred and mixed, and subsequently caused to solidify.
Specifically, the method for manufacturing a composite metal member disclosed in JP 06-238422 A is characterized in that a reinforcing material (a ceramic material) is admixed with a metal material in a semi-molten state.
According to the composite forming method disclosed in JP 2004-136363 A, a melted low-melting metal material is cooled to a semi-molten state having a thixotropic property in which a liquid phase and a solid phase are both present. The low-melting metal material is kneaded in this state with a carbon nanomaterial, and a composite material is obtained. A metal forming machine provided with heating means is used to inject the composite material into a die while maintaining the thixotropic property; and an article formed from composite metal is formed using the die.
Specifically, the composite forming method disclosed in JP 2004-136363 A is characterized in that a reinforcing material (carbon nanomaterial) is kneaded with a low-melting metal material in a semi-molten state.
In either method the metal material is in a semi-molten state; therefore, movement of the added reinforcing material is limited, and aggregation of the reinforcing material can be prevented. As a result, the reinforcing material can be uniformly distributed on the metal material.
However, since the metal material is in a semi-molten state, the added reinforcing material moves at a slow rate, and some time is required before the reinforcing material is moved to all parts of the metal material.
In processes of this type, stirring is generally performed in the presence of argon gas or another inert gas in order to prevent the metal material from oxidizing. Increasing the stirring time heightens the risk that the inert gas will be entrained in the metal material, and leads to a deterioration in the quality of the formed article.